The diagnostic I see is:

tmp.swift:3:19: error: overriding non-open instance method outside of its defining module
    override func hash(into hasher: inout Hasher) { }
                  ^
tmp.swift:3:19: error: overriding declarations in extensions is not supported
    override func hash(into hasher: inout Hasher) { }
                  ^
ObjectiveC.NSObject:4:17: note: overridden declaration is here
    public func hash(into hasher: inout Hasher)
                ^

I think the first error kinda' explains what the issue is, but I agree that the extension part of the diagnostic doesn't make sense by looking at the code.

@swift-ci create

Comment by Minhyuk Kim (JIRA)

I created a PR here: #35465

This PR will provide a nice diagnostic for this common corner-case, but I do believe we need a general purpose fixup to the wording of the original diagnostic to fully resolve this. I'd like to see something like

overriding declarations in extensions is not supported

become

method 'hash(into:)' is declared in an extension of 'NSObject' and cannot be overridden

Ideally if we could emit a note that points to the declaration itself (we have a generic note for this: drag::kind_declared_here) that would be cool too.

Comment by Ken Harris (JIRA)

Ooooh. I was all ready to say that that's a separate issue, and then I re-read everything.

I just now realized what the original error message is trying to say. As written, the "extension" message is somewhat ambiguous, and my mind was focused on getting my program to work, not on picking apart compiler error messages. In hindsight, my first interpretation was obviously nonsensical, and the other possible interpretation (which I didn't even consider at the time) is the correct one.

Robert's alternate phrasing is terrific, and I fully support that. If the error message were changed to that, I would consider this issue resolved.

Minhyuk's helpful note for the "hash" case, in addition to the normal error message, would be a great addition, too.

Comment by Minhyuk Kim (JIRA)

Thanks for the great opinion @CodaFi!

As you suggested, I modified existing warning to:

`method 'foo()' is declared in extension of 'A' and cannot be overriden`

and

`method 'foo()' declared in 'A' cannot be overriden from extension`

I kept the special handling for NSObject.hash method like this for additional clarify for users:

`NSObject.hash(into:)` is not overridable; subclasses can customize hashing by overriding the `hash` property